It is well known in the art that the reaction transmitted from the steered road wheels of an automotive vehicle to the driver's manipulative effort applied to the steering wheel when the road wheels are being steered increases with a decrease in the vehicle speed and peaks when the vehicle is held at a halt. A power-assisted steering system of an automotive vehicle has thus been developed principally with a view to permitting a vehicle driver to maneuver the steering system with a reduced manipulative effort at relatively low vehicle speeds or when the vehicle is held at a halt. As a corollary of such an intrinsic function of a power-assisted steering system, the steering wheel tends to become so unresisting to the driver's effort applied to the wheel as to cause the vehicle to be oversteered or otherwise make an unintended or unwanted turn that would result in a serious accident. When, furthermore, the driver of a vehicle intends to steer the vehicle through a small angle in an attempt to adjust the direction of travel of the vehicle so as to recover a lateral slip of the vehicle under the influence of a sidewind, the steering wheel tends to be turned through an unnecessarily large angle and may cause the vehicle to wobble.
An advanced version of a power-assisted steering system has therefore been proposed and put to practical use which is adapted to produce reduced hydraulic steering assistance at medium-to-high vehicle speeds and increased hydraulic steering assistance at low-to-medium vehicle speeds, a typical example of such a power-assisted steering system being disclosed in Japanese Patent Publication No. 51-41736. The steering system therein taught has a fluid-operated steering power cylinder assembly in which two pressure-acting chambers are provided across a power piston and communicate with each other through a bypass passageway. The bypass passageway thus provided between the two pressure-acting chambers is arranged with a servo valve adapted to vary the flow rate of fluid therethrough in response to changes in vehicle speed. When the vehicle is cruising at relatively high speeds, the servo valve is conditioned to provide relatively high flow rates of fluid therethrough so that the pressurized fluid introduced into one of the pressure-acting chambers is circulated to the other pressure-acting chamber at rates dependent on the vehicle speeds. This causes reduction in the fluid pressure in the former fluid-acting chamber and accordingly in the hydraulic steering assistance achieved by the cylinder assembly.
A drawback is encountered in a prior-art power-assisted steering system of the above described nature in that the flow rate of fluid through the servo valve is subjected to change due to the difference between the respective fluid pressures in the pressure-acting chambers. Thus, the differential pressure effective between the pressure-acting chambers is affected by the flow rate of fluid through the servo valve and for this reason can not be controlled with accuracy, making it impossible to precisely control the steering effort to be achieved by the steering system. Because, furthermore, of the fact that only a limited differential pressure is in play between the two pressure-acting chambers when the vehicle is being steered through a small angle to recover the lateral slip of the vehicle as caused by a sidewind, the differential pressure between the pressure-acting chambers and accordingly the steering effort achieved by the steering system can not be properly controlled in response to changes in the vehicle speed. The present invention contemplates provision of an improved fluid-operated control apparatus for a steering system overcoming these drawbacks of a prior-art power-assisted steering system of the described general nature.